1. Field of the Invention
The present invention relates to a magnetostriction type torque sensor which detects magnetic property change of magnetic property change material which is attached around a shaft.
2. Description of the Related Art
In an electric power steering device installed in a vehicle, a torque sensor detects torque which is given to a steering shaft by driver's steering. Thus, steering assist power is provided depending on torque signals output from the torque sensor. A magnetostriction type torque sensor is disclosed as such a torque sensor (See JP2004-239652A and JP2005-331453A). In this technology, a magnetostriction film having magnetic anisotropy is attached on a surface of the shaft. Then, the torque is applied to the shaft from the outside so that magnetic permeability change in the magnetostriction film corresponding to twisting force is detected as change in an impedance property of a detection coil. Moreover, in technologies disclosed in JP2005-331453A and JP2005-321316A, a pulse of excitation voltage is applied so that the change in the impedance property of the detection coil is detected. In the technologies, the pulse of the excitation voltage is applied so that a maximum of increasing current which flows in the detection coil is limited within a predetermined range.